I thought that it's North for entirety of the thing located in the North and Northern for only the northern part of a bigger whole, but this is not always the case: Northern Hemisphere, Ohio Northern University, North Seattle Community College, North Africa.

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the question linked to doesn't answer when is North preferred as adjective. Why not Northern Seattle Community College?
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SF.Jul 21 '12 at 6:57

The terms you're asking about are simply names chosen by institutions. They could have just as easily chosen The University of Northern Ohio, The University Located Centrally in the Northern part of Ohio - next to the big tree, or any other number of combinations of words. When it comes to the names of institutions, it's 100% marketing. For correct usage of capitalization, see my answer below.
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Zac BrownJul 21 '12 at 14:42

4 Answers
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It's largely (but not completely) arbitrary whether to use, say, North or Northern as a modifier.

In contexts where North X is a well-recognised "entity" (North Korea, North America, for example), there's a very strong tendency to use the shorter North.

In contexts where there's more the sense of an "ad-hoc" distinction being made between the geographically-distinct areas within "X", the tendency is to use the more overtly adjectival form. Thus we're likely to refer to "Northern Alpine slopes", or Northern England, because these areas are still fundamentally perceived as being part of "the Alpine slopes", or "England", rather than autonomous entities.

But as been mentioned in comments, in many cases (names of institutions, for example), it's entirely a matter for the people in charge what they call themselves. They may have reasons for their choice which have little or no relevance to how others see things.

North is an area; northern is originating from, or having the characteristics of, the north.

So, North England or North Seattle are areas, in the north of England or Seattle respectively. Lancashire Hotpot or Black Pudding are northern English foods, as they originate from North England, and "By 'eck, it's proper grand..." is a northern English comment, as it is characteristic of the north.

For your college, if it has one campus in north Seattle, it is North Seattle Community College. If it has more than one campus, you could call it Northern (because its characteristic is being in the north)Seattle Community College to distinguish it from the Southern/Eastern/Western Seattle Community College Campuses.

I'm not satisfied with your explanation of why the campus uses "North". Why is it the University of North Alabama, but the University of Northern Colorado? Eastern Connecticut State University, but East Tennessee State University? University of West Georgia, but Western Oregon University? Seems these examples might be cases where either term could be used? (Don't worry, I won't even bother asking you to explain the name of this institution...)
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J.R.Jul 21 '12 at 10:25

Terms that denote regions of the world or of a particular country are often capitalized, as are a few of the adjectives and nouns derived from such terms. The following examples illustrate not only the principles sketched in 8.1 but also variations based on context and usage. For terms not included here or for which no suitable analogy can be made, consult Webster’s or an encyclopedia: if an otherwise generic term is not listed there (either capitalized or, for dictionary entries, with the indication capitalized next to the applicable subentry), opt for lowercase. Note that exceptions based on specific regional, political, or historical contexts are inevitable and that an author’s strong preference should usually be respected. See also 8.45.

the Swiss Alps; the Australian Alps; the Alps; an Alpine village (if in the European or Australian Alps); Alpine skiing; but alpine pastures in the Rockies (see also 8.52)
Antarctica; the Antarctic Circle; the Antarctic Continent
the Arctic; the Arctic Circle; Arctic waters; a mass of Arctic air (but lowercased when used metaphorically, as in “an arctic stare”; see 8.60)
Central America, Central American countries; central Asia; central Illinois; central France; central Europe (but Central Europe when referring to the political division of World War I)
the continental United States; the continent of Europe; but on the Continent (used to denote mainland Europe); Continental cuisine; but continental breakfast
the East, eastern, an easterner (referring to the eastern part of the United States or other country); the Eastern Seaboard (or Atlantic Seaboard), East Coast (referring to the eastern United States); the East, the Far East, Eastern (referring to the Orient and Asian culture); the Middle East (or, formerly more common, the Near East), Middle Eastern (referring to Iran, Iraq, etc.); the Eastern Hemisphere; eastern Europe (but Eastern Europe when referring to the post–World War II division of Europe); east, eastern, eastward, to the east (directions)
the equator; equatorial climate; the Equatorial Current; Equatorial Guinea (formerly Spanish Guinea)
the Great Plains; the northern plains; the plains (but Plains Indians)
the Midwest, midwestern, a midwesterner (as of the United States)
the North, northern, a northerner (of a country); the North, Northern, Northerner (in American Civil War contexts); Northern California; North Africa, North African countries, in northern Africa; North America, North American, the North American continent; the North Atlantic, a northern Atlantic route; the Northern Hemisphere; the Far North; north, northern, northward, to the north (directions)
the Northeast, the Northwest, northwestern, northeastern, a northwesterner, a northeasterner (as of the United States); the Pacific Northwest; the Northwest Passage
the poles; the North Pole; the North Polar ice cap; the South Pole; polar regions (see also Antarctica; the Arctic)
the South, southern, a southerner (of a country); the South, Southern, a Southerner (in American Civil War contexts); the Deep South; Southern California; the South of France (region); Southeast Asia; South Africa, South African (referring to the Republic of South Africa); southern Africa (referring to the southern part of the continent); south, southern, southward, to the south (directions)
the Southeast, the Southwest, southeastern, southwestern, a southeasterner, a southwesterner (as of the United States)
the tropics, tropical; the Tropic of Cancer; the Neotropics, Neotropical (of the New World biogeographical region); the subtropics, subtropical
the Upper Peninsula (of Michigan); the upper reaches of the Thames
the West, western, a westerner (of a country); the West Coast; the West, Western (referring to the culture of the Occident, or Europe and the Western Hemisphere); west, western, westward, to the west (directions)

and from 8.45: Points of the compass Compass points and terms derived from them are lowercased if they simply indicate direction or location. But see 8.46. pointing toward the north; a north wind; a northern climate to fly east; an eastward move; in the southwest of France; southwesterly
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Zac BrownJul 21 '12 at 14:38